1. Field of the Invention
The present disclosure relates to liquid crystal display technology, and more particularly to a LED backlight source for liquid crystal devices (LCDs) and the LCDs.
2. Discussion of the Related Art
With the development of the display technology, the backlight solutions have been greatly enhanced. CCFL is one of the conventional backlight sources of LCDs. However, as the CCFL backlight is characterized by attributes including low color reduction ability, low lighting efficiency, high discharging voltage, bad discharging characteristics in low temperature, and also, the CCFL needs a long time to achieve a stable gray scale, LED backlight source is a newly developed technology.
FIG. 1 is a circuit diagram of one conventional LED backlight source of LCDs. As shown, the LED backlight source includes a boost circuit, a backlight driving control circuit, and a LED string. The boost circuit includes an inductor (L), a rectifier diode (D1), a first MOS transistor (Q1), a capacitor (C), and a first resistor (R1). One end of the inductor (L) receives the input voltage (Vin). The other end of the inductor (L) connects to a positive end of the rectifier diode (D1) and to a drain of the first MOS transistor (Q1). A gate of the first MOS transistor (Q1) is driven by a first square-wave signals (PWM1) provided by the backlight driving control circuit. A source of the first MOS transistor (Q1) is electrically grounded via the first resistor (R1). A negative end of the rectifier diode (D1) receives the output voltage (Vout) and provides the output voltage (Vout) to the LED string. The negative end of the rectifier diode (D1) is electrically grounded via the capacitor (C). The negative end of the LED string also connects to a second MOS transistor (Q2). The drain of the second MOS transistor (Q2) connects to the negative end of the LED string. The source of the second MOS transistor (Q2) is electrically grounded via a third resistor (R3). A gate of the second MOS transistor (Q2) is driven by a second square-wave signals (PWM2) provided by the backlight driving control circuit. The operation current of the LED string may be increased or decreased by changing the duty-cycle ratio of the second square-wave signals (PWM2). In addition, the backlight driving control circuit also includes a second resistor (R2) for ensuring the driving frequency of the first square-wave signals (PWM1) provided to the gate of the first MOS transistor (Q1).
When the power supply of the LCD is cut off, the input voltage (Vin) of the LED backlight source is decreased until being equal to zero. The backlight driving control circuit may not terminate its operation immediately due to the capacitor and a larger power supply range of the backlight driving control circuit. However, during this time period, the output voltage (Vout) of the LED string remains the same. It may cause the duty-cycle ratio of the first square-wave signals (PWM1) provided to the gate of the first MOS transistor (Q1) may increase as the input voltage (Vin) has been decreased. For this reason, the energy-saving time (D×T) within the operation period (T) of the first square-wave signals (PWM1) for the inductor (L) is increased, which results in an increased maximum current for the inductor (L) and an increased input current (Iin) of the LED backlight source. As such, The sharply input current may damage the electrical components with in the LED backlight source.